Example embodiments disclosed herein pertain to current limiters, i.e. circuit breakers. More particularly, example embodiments disclosed herein pertain to current limiters in an environment which may induce transient surges.
FIG. 1 is a schematic diagram wherein a prior art telecommunications power card 2 is depicted. The purpose of telecommunications power card 2 is to provide constant voltage to a load that is connected to it.
On the exterior of the card, common node 4 and negative VIN node 6 are disposed to connect to a voltage source providing, for example, −48 volts. The term “node” refers to any equipotential point in an electrical device, either at a terminus, or not at a terminus. Voltage sources of −48 volts are widely used in telecommunications applications; nearly all telephone central offices are powered from −48 volt sources wherein the “hot” side of the supply is negative with respect to ground.
Hot swap controller 8 is disposed on prior art telecommunications power card 2, and is coupled to positive ground node 4 and negative VIN node 6 from which it derives approximately −48 volts when connected to an active power source providing −48 volts. Hot swapping (also known as hot plugging) is the ability to add, remove and replace components of a system while the system is powered. In the context of telecommunications applications, the hot swapping of power sources, such as batteries or power supplies, is commonly performed.
Capacitor 10 is electrically coupled to hot swap controller 8 at nodes 12 and 14. Increases of voltage correspondingly store excess charge in the capacitor, thus moderating the change in output voltage and current induced by a transient power event.
Since it is desirable to provide less than the −48V input for a load that is coupled to telecommunications power card 2, (e.g. +3.3 volts), a DC to DC converter 16 is provided on telecommunications power card 2 and is electrically coupled to hot swap controller 8 at nodes 12 and 14. Thus, capacitor 10 and DC to DC converter 16 are arranged in parallel fashion. DC to DC converter 16 is a load with respect to the power output of hot swap controller 8.
In electrical engineering, a DC to DC converter is a circuit which converts a source of direct current from one voltage to another. A DC to DC converter may serve to isolate an input power source (i.e. hot swap controller 8 from the load connected to the output of the DC to DC converter at common node 18 and VOUT node 20. DC to DC converters are commercially available as blocks or modules from Datel, Power Trends, Integrated Power Designs, or others.
FIG. 2 is a schematic diagram which depicts one form of a current limiter 22 of the prior art. Current limiter 22 is coupled to common node 4 and negative VIN node 6. These nodes provide power to the current limiter 22 which in turn provides power to the other components of hot swap controller 8 via VOUT node 24 and common node 4. Current source 26 is coupled to common node 4, and provides a constant reference current. It is comprised of voltage reference source 28, resistor R1, amplifier 30, and transistor 32 which are arranged in a well known configuration to provide constant current I1 based on the reference voltage provided by voltage reference source 28. Note that in FIG. 2, voltages are referred to −VIN at node 6.
Resistor R2 is coupled to current source 26 at node 34 and to negative VIN node 6. The constant current provided by current source 26 flows through resistor R2 producing reference voltage V1 across resistor R2 such that:V1=I1·R2
Resistor RSENSE is coupled to negative VIN node 6 and to pass transistor 36 which acts as a circuit breaker at node 38. When the circuit is not broken by pass transistor 36, current flows between negative VIN node 6 and VOUT node 24. The purpose of resistor RSENSE is to sense the current I2 flowing between negative VIN node 6 and VOUT node 24 and produce a voltage VSENSE at node 38 such that:VSENSE=I2·RSENSE 
Comparator 40 is coupled to node 34 and to node 38 such that it compares voltage VSENSE to voltage V1. The output of comparator 40 is coupled to latch 42 indicating when the circuit should be broken or modified. During normal operation, VSENSE is less than V1, so comparator 40 signals that the circuit should not be broken. When VSENSE exceeds V1, comparator 40 signals that the circuit should be broken or modified in some way.
Constant current I1 is only a small fraction of the current limit. R2 can be chosen to produce the reference voltage corresponding to the desired limit for current I2 as sensed by resistor RSENSE. Since the resistance value of R2 is multiplied by constant current I1, higher reference voltages can be achieved by increasing the resistance value of R2.
Latch 42 is coupled to the output of comparator 40 and to pass transistor 36. When comparator 40 signals that the circuit should be broken, latch 42 propogates the signal to pass transistor 36, breaking the circuit. Latch 42 maintains the signal to break the circuit until receiving a reset signal via reset node 44. As will be appreciated by those skilled in the art, there are many alternatives to Latch 42 for various applications.
FIG. 3 is a graph depicting a constant-current threshold current limiter of the prior art. In FIG. 3, the vertical axis indicates current level and the horizontal axis indicates time increasing from left to right. The dashed line labeled ICB represents a constant-current threshold in common use. When this threshold is exceeded, it is an indication that the circuit breaker must interrupt current flow between the power source and the load. The solid line in FIG. 3 depicts an example of a current level which is initially constant. At a certain point in time, the current level suddenly surges to a dramatically higher level, which is above the constant current limit.
Since the current level exceeds the constant-current threshold, the circuit is broken. In some cases, this is undesirable. For example, if an additional power source is brought online, the voltage of the combined power sources may suddenly increase. Since a smoothing capacitor is typically coupled in parallel with the power supply and the load, the capacitor begins charging to the new voltage. The transient increase in current caused by the capacitor charging to the new voltage causes the circuit to break prematurely. Although the current surge may be relatively brief, and the current level decays exponentially as the capacitor approaches the new voltage, it can cause premature circuit breakage because the design of the prior art constant-current threshold circuit breaker is unforgiving.
One approach to this problem is to provide a current limiter which is temporarily disabled, for a fixed amount of time, to allow for a transient surge. This approach has the disadvantage that it does not protect the load from a damaging current surge such as that induced by a lightning strike or other dramatic power event. What is needed is a current limiter which is continuously online in order to protect the load, but which handles the transient surge caused by bringing a new or additional power source online; the prior art fails to properly address this problem
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.